This invention relates generally to scintillator materials useful for radiation detectors and for radiation detectors made therewith, and more particularly to scintillator materials that can be used in thin slices and/or processed at a relatively low temperature, and radiation detectors made therewith.
Solid state scintillator materials have long been used as radiation detectors to detect penetrating radiation in such applications as x ray counters and image intensifiers. The scintillator materials emit visible or near visible radiation when stimulated by x rays or other high energy electromagnetic photons. In typical medical or industrial applications, the optical output from the scintillator is directed to a photoelectrically responsive device to produce electrical output signals, where the amplitude of the signals is proportional to the deposited energy. The electrical signals can than be digitized by a computer for display on a screen or other permanent medium. Such detectors play an important role in computerized tomography (CT) scanners, digital radiography (DR), and other x ray, gamma radiation, ultraviolet radiation, and nuclear radiation detecting applications. In medical applications, it is especially desirable that the scintillator efficiently absorb nearly all the x rays that pass through a patient, so that the detector utilizes a maximal amount of the high energy administered, and the patient is not subject to a higher radiation dose than necessary.
Among the preferred scintillator compositions in the present generation of CT scanners are ceramic scintillators that employ at least one of the oxides of lutetium, yttrium, and gadolinium as matrix materials. These are described in detail, for example, in U.S. Pat. Nos. 4,421,671, 4,473,513, 4,525,628, and 4,783,596. These scintillators typically comprise a major proportion of yttria (Y2O3), up to about 50 mole percent gadolinia (Gd2O3), and a minor activating proportion (typically about 0.02-12, preferably about 1-6 and most preferably about 3 mole percent) of a rare earth activator oxide. Suitable activator oxides, as described in the aforementioned patents, include the oxides of europium, neodymium, ytterbium, dysprosium, terbium, and praseodymium. Europium-activated scintillators are often preferred in commercial X ray detectors because of their high luminescent efficiency, low afterglow level, and other favorable characteristics.
Another important consideration for scintillators is to reduce damage that occurs to the scintillator upon repeated exposure of the scintillator to high energy radiation. Radiographic equipment employing solid state scintillator materials for the conversion of high energy radiation to an optical image may experience changes in efficiency after exposure of the scintillator to high dosages of radiation. For example, radiation damage for bismuth germanate single crystal scintillators may be as high as 11% after a thirty minute exposure to ultraviolet radiation from a mercury lamp. Similar results are reported for higher energy gamma radiation. Furthermore, the variation in radiation damage from crystal to crystal of bismuth germanate scintillators is high, approximating a factor of at least 30. A similar change in efficiency can be found when polycrystalline type ceramic scintillators are exposed to high energy radiation dosages.
Radiation damage in scintillators is characterized by a change in light output and/or a darkening in color of the scintillator body with prolonged exposure to radiation. Radiation damage can lead to “ghost images” from prior scans which thereby reduce image resolution. The change in light output that occurs upon radiation damage is often found to be variable in magnitude from batch-to-batch of the same scintillator, making it difficult to predict how any individual scintillator will change over time and thus, making it difficult to implement quantitative correction measures. For example, yttria-gadolinia ceramic scintillators activated with europium exhibit a reduction in light output of 4 to 33%, depending upon the scintillator batch, for 450 roentgens of 140 kVP x rays. This amount of variation in light output which can occur as a result of x ray damage is undesirable in a quantitative x ray detector.
Moreover current non-water soluble scintillator materials are difficult to manufacture, owing to their high melting points and the thickness of the scintillator coating that has to be applied to stop X-rays from reaching and damaging the detector. The thick coatings also result in inefficient optical transmission.